groupby

itertools.groupby(iterable[, key])

Warning

  • sorted the data using the same key function

    The operation of groupby() is similar to the uniq filter in Unix. It generates a break or new group every time the value of the key function changes (which is why it is usually necessary to have sorted the data using the same key function). That behavior differs from SQL’s GROUP BY which aggregates common elements regardless of their input order.

Example

  • Question

    • Input

      [{'lv1': 1, 'lv2': 2, 'name': 'table1'},
       {'lv1': 1, 'lv2': 2, 'name': 'table2'},
       {'lv1': 1, 'lv2': 2, 'name': 'table3'},
       {'lv1': 1, 'lv2': 3, 'name': 'table3'},
       {'lv1': 1, 'lv2': 3, 'name': 'table5'},
       {'lv1': 2, 'lv2': 1, 'name': 'table1'},
       {'lv1': 2, 'lv2': 1, 'name': 'table2'}]
      
    • Output

      [{'lv1': 1, 'lv2': 2, 'name': ['table1', 'table2', 'table3']},
       {'lv1': 1, 'lv2': 3, 'name': ['table3', 'table5']},
       {'lv1': 2, 'lv2': 1, 'name': ['table1', 'table2']}]
      
  • Answer

    In [3]: import itertools
    
    In [4]: nodes = [{'lv1': 1, 'lv2': 2, 'name': 'table1'},
       ...:  {'lv1': 1, 'lv2': 2, 'name': 'table2'},
       ...:  {'lv1': 1, 'lv2': 2, 'name': 'table3'},
       ...:  {'lv1': 1, 'lv2': 3, 'name': 'table3'},
       ...:  {'lv1': 1, 'lv2': 3, 'name': 'table5'},
       ...:  {'lv1': 2, 'lv2': 1, 'name': 'table1'},
       ...:  {'lv1': 2, 'lv2': 1, 'name': 'table2'}]
    
    In [5]: map(lambda x : {'lv1': x[0][0], 'lv2' : x[0][1], 'name': [y['name'] for y in x[1]]}, itertools.groupby(sorted(nodes, key=lambda x : (x['lv1'], x['lv2'])), lambda x : (x['lv1'], x['lv2'])))
    Out[5]:
    [{'lv1': 1, 'lv2': 2, 'name': ['table1', 'table2', 'table3']},
     {'lv1': 1, 'lv2': 3, 'name': ['table3', 'table5']},
     {'lv1': 2, 'lv2': 1, 'name': ['table1', 'table2']}]
    

References

[1] [email protected], 9.7. itertools — Functions creating iterators for efficient looping

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